A proliferation of tape recorders for live music, dictation, and video signals of films and television broadcasts has recently taken place. This has been augmented by computers and word processors, which require software in the form of floppy disks or hard disks. These are conventionally prepared from liquid dispersions of magnetic material. In order to achieve well-suspended magnetic particles, the dispersion has been spun onto disks to obtain a thin coating. While the coating was still wet, the coated disk has been placed in a magnetic field to orient the magnetic particles. Then the disk was cured, usually by a complex heat cycle. The yield from this method has been very poor.
It has been suggested by the prior art that magnetic material, such as finely divided ferric oxide particles, may be suspended in water and deposited on a tape by electrophoresis. We have found that other liquid carriers with low dielectric constants and high resistivities--in particular, ISOPARs--are superior materials for this purpose. In order for electrophoresis to take place, the toner particles must be charged with the right polarity. This is the office of the charge director. The liquid toner bath, consisting of ISOPAR plus toner particles and charge director, must be nearly electrically neutral. Thus, the charge director not only introduces charges on the toner particles, but also equal numbers of opposite charges associated with particles, called counter ions. The counter ions may be charged individual molecular ions, chain polymers, micells, or other molecular agglomerates. There are two possible ways a charge director can serve its function. In the first, the charge director consists of neutral entities until they are mixed with the toner particles. Then, due to enthalpy differences between the charge director and the toner particles, which induce a chemical reaction, the toner takes one sign of charge and the charge-director molecules the other. This can occur because electrons exchange between the two types of particles or because an ionic molecular group transfers from the charge-director molecules to the toner particles. The second and more common way in which a charge director functions is that it is an ionic solution in the liquid carrier before it is mixed with toner particles. Then one of the ionic species preferentially attaches to the toner particles when the toner and the charge director are mixed. The sign of the ion that attaches to the toner depends in general on the surface properties of the toner particles and the ionic species in the charge director. A given charge director may charge one toner material positively and another negatively or not at all. The preferred arrangement is to have the larger of the two ionic species attach to the toner so the smaller, more mobile, one serves as the counter ion.
If the toner suspension is under-charge-directed or the ion attachment probability is low so there is on average less than one charge-director ion (of the right sign) attached per toner particle, then there is a tendency for the toner particles to form floccules. The mechanism responsible for this flocculation arises because a neutral toner particle adjacent to a charged one will experience an induced dipole that results in an attractive force, causing the particles to bind together.
The toner particles in the bath will all reach the same equilibrium chemical potential. In equilibrium, the charge on a given particle will be larger if it has a larger radius--because its capacitance is inversely proportional to the radius--and if the number of surface sites to which charge-director ions attach (called "hooks") is larger. The viscous drag on a particle in a liquid is also proportional to its radius. Therefore, the mobility, which is proportional to the charge per particle and inversely proportional to the viscous drag, will tend to be independent of particle size if the density of surface hooks is the same on all particles. This means that particles with uncontrolled hook densities will respond differently to the fields, tending to produce non-uniform deposits of toner. The result is that the toner particles having the largest mobilities will move out of the developing liquid first, leaving the sluggards behind. As this progresses, even if the bath is replenished to maintain a constant toner-particle density, particle deposit in subsequent applications of the developing dispersion will become less dense and the dispersion is said to be "exhausted".
If a toner suspension is over-charge-directed, in the second case where it is an ionic solution, then there may be extra ions in the bath with the same sign as the toner particles. This is described in the art as a circumstance where the toner has "continuous phase conductivity". When there is continuous phase conductivity, the ions with the same sign as the toner particles compete with them to reach the image area. The result is a lower toner density reaches the charged substrate than optimum. The continuous phase conductivity also has an adverse effect on the efficiency of toner transfer from a surface onto which it is originally deposited to a carrier in a printing application.
We have discovered that it is possible to produce substantially homogeneous films of ferromagnetic particles, such as ferric oxide, by encapsulating them in a thermoplastic resin. The resin must be such that it is insoluble in the liquid dispersant, such as ISOPAR (trademark for a series of low-boiling isomerized aliphatic hydrocarbon liquids manufactured by Exxon Corporation). Furthermore, the coating, or encapsulant, must be such that it will melt after deposit to yield a smooth surface and permit orientation of the ferromagnetic particles. In addition, the surface of the encapsulant should have a plurality of functional sites to accommodate a charge director. This enables us to produce encapsulated ferromagnetic particles, such as ferric oxide, which are controllably charged and have the same mobilities. Thus, the field introduced to cause electrophoresis will drive the electrophoretically mobile toner particles into place without exhausting the suspension. We may consider the relationship of the encapsulant and the charge director as being analogous to ion-exchange or acid-base phenomena.